In a magnetic disk drive, an actuator mounted with a magnetic head at one end is rotationally driven around a pivot axis as a rotational axis of the actuator, thereby the magnetic head is positioned in an optional radial position on a magnetic disk to perform write and read. A coil is mounted on the actuator at the other end, and a voice coil motor fixed to a housing of the magnetic disk drive generates drive force for rotating the actuator. The voice coil motor is configured by a magnet for generating magnetic flux, and yokes made from a soft magnetic material for forming a magnetic circuit.
As an example of the voice coil motor,. for example, Japanese Patent Publication No. 5-122917 (“patent document 1”) discloses a configuration where an auxiliary yoke is equipped in a portion of at least one side end face of the magnetic in order to effectively suppress leakage flux flowing to an external component.
The voice coil motor essentially generates force for rotating the actuator in a direction parallel to a coil plane, but sometimes induces force in a direction perpendicular to the coil plane, that is, in a direction parallel to a pivot axis due to a configuration or shape of the voice coil motor, or variation in assembly. The force is called “out-of-plane excitation force” because it works in a direction perpendicular to a moving plane of the coil. The out-of-plane excitation force is induced by a phenomenon that a magnetic field applied from the magnet to the coil has not only a component perpendicular to the coil plane, but also a component parallel to the coil plane. In particular, the out-of-plane excitation force becomes large in a voice coil motor configured by using only one magnet in order to reduce cost.
When the out-of-plane excitation force is large, the coil, a part of the actuator, vertically vibrates, which in turn causes slight vibration of a magnetic head. This results in reduction in positioning accuracy of the magnetic head onto a predetermined recording track, which is a factor of hindering high capacity of a magnetic disk drive.
The technique described in the patent document 1 includes a structure for suppressing the leakage flux flowing to an external component from the voice coil motor, which may not solve the problem of the out-of-plane excitation force generated in the voice coil motor.